Your choices so far:
1 Comfortable indoor climate; 2 Sunshine
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | District cooling | 1 Comfortable indoor climate |
| Biomass (fermentable sludge) | District heating | Electricity |
| Biomass (solid) | Electricity | Process cooling (< 0 °C) |
| Geothermal | Fuel: Gaseous | Process heat/steam (50 - 150 °C) |
| 2 Sunshine | Fuel: Liquid | Process heat (150 - 1000 °C) |
| Water | Fuel: Solid | Process heat (> 1000 °C) |
| Wind | Local cooling (ind. house) | Transport |
| Residual oils/fats etc | Local heating (ind. house) |
Systems for the use of solar energy for small-scale heating purposes are commonly separated into two different types:
- Passive systems: These are usually systems for low-temperature house heating where there are no special collectors for the solar energy but where the building architecture in itself provides the heating. Windows, extended roofs and – in some cases – massive building elements to store heat may be parts of such a system. But passive solar heating is not only to plan for solar energy to enter the building – it is also to hinder it when it is excessive and to maintain it in the building to the extent that is desired. The energy service is to provide a comfortable indoor climate – not to accumulate the most solar energy.
- Active systems: Active solar heating involves concentrating the irradiation by aid of mirrors, lenses or specially designed collectors to provide higher temperatures. But these systems will usually still be designed for space or tap-water heating. Also heat pumps may be used to achieve higher temperatures and such systems would also be classified as active.
In the larger scale, solar heating may well provide a significant contribution to district heating systems: With a solar panel, collection efficiencies well in excess of 90% are common and supposing then that the panels are oriented at a right angle to the sun and that the incident radiation is 800 W/m2 one may collect a bit more than 700 W/m2. Further assuming that the district heating system needs 7 MW we find that 10 000 m2 of solar collectors will be needed to produce the thermal power decided. The simple, flat-plate type of solar collector is also quite cheap. So even though solar heating systems for district heat applications thus tend to become quite big they may still serve as a complement to easily controllable, preferably fuel-fired heating.
Solar energy is in itself not well suited for the production of district cooling. However, if solar heat production is integrated in a district heating system large enough to host tri-generation, then part of the cooling supplied will actually originate from solar energy.